Camera trick lets you see sound waves in falling water

From this still image you’d think the hose dispensing the water is being moved back and forth. But watch the video after the break and you’ll see the hose is quite steady, as is the standing wave of water. It’s bizarre to be sure. Knowing how it works makes cognitive sense, but doesn’t really diminish the novelty of the demonstration.

We’d call it an optical illusion but it’s really more of a technological illusion. The water is falling past a sub-woofer speaker which is tuned to 24 Hz. At the same time, the camera filming the demonstration is capturing 24 frames per second. As was mentioned then, it’s much like flashing a light to freeze the water in mid-air. But the flashing of the frames is what causes this effect.

27 thoughts on “Camera trick lets you see sound waves in falling water”

No, it’s exactly opposite. The rolling shutter CMOS would ruin the illusion by capturing parts of the frame at a different time, whereas CCD captures the whole frame at a single point of the oscillation and that’s what makes the water appear stationary.

I was just about to comment that you couldn’t do this with any cellphone camera, or a DSLR, or a cheap pocket camera, because they all use CMOS sensors. You need a real DV camera, or a midrange compact camera to pull it off.

The thing about CCD is that it requires more bandwidth out of the hardware, because of how it works. It’s a charge coupled device, which in practice means that each line of pixels is coupled in series like a bucket chain where the accumulated charge travels from one pixel to the next as the clock line is pulsed. It’s designed for television where each line is pulsed in turn to produce the analog (!) scanlines directly. This is also the reason why CCD sensors get that purple shaft of light artifact as a reaction to bright lights.

The way the CMOS rolling shutter works is by rolling a zero line ahead to keep the exposure time constant. You could read a CCD like a CMOS sensor, but since you can’t blank the CCD cells at will without closing the shutter and clocking in an empty line, you’d get uneven exposure.

You have to either read the CCD sensor line by line continously in a loop like shooting a video to keep the timing constant, or you dump everything at one go in parallel. The easiest way to do that is by having a second CCD right next to the one being exposed, except covered with a mask, to act as a really fast buffer where the camera dumps the image, and then clocks it out slower to read it.

I think the effect is most dramatic when there is a person in the frame moving at normal speed along with the water flow. It makes you think it’s not just slow motion or other trickery, even though it is.

I don’t think so. Eyes have a different way of sampling the light that enters them. IIRC, the “frame rate” is more like 4Hz, but we have brains that compensate for motion and stuff. Don’t quote me on that. However, you can see the same effect by using light that flashes at a certain frequency, like some fluorescent lights and street lamps, which usually pulse at their driving frequency (or double). You can sometimes see this kind of stuff at night due to that…

“frame rate” is a term that should probably be eternally forbidden when referring to (human) eyes in a general sense. Our eyes integrate (average) continuously over time, and if you flash a light fast enough, it just looks solid (flashes blur together). This frequency isn’t the same for all people, and it’s not even the same for the center of your vision vs the periphery. It’s WAY more complicated than just saying “we see at x fps”.

For experiencing fluid motion, human eyes can easily differentiate between 60hz and 120hz update rates. If your visual field propagated to your brain at 4hz, you’d be *screwed*.

When you see flicker with a fluorescent bulb or a (badly designed) LED lamp, it’s almost surely because the flicker frequency is too low.

This effect works with a simple strobe light as well. There’s a demonstration in a hallway here at mit where colored water is dropped with a user-controllable strobe light. When single drops fall it seems like you can reverse time by changing the strobe-rate to just the right value. The drop will appear to fall upwards in precise reverse motion. It’s a really stunning thing to see up close with your own eyes (though the flashing can distract).

One can do this trick without any camera. Place the hose in front of a black panel in dim light and use a strobe ligth flashing at same frequency than the subwoofer and the phenomenom will appear to your eyes as this video.

You can get a similar effect when recording the excursion of a subwoofer around the frame rate of a camera. :)
all kinds of strange things can appear to happen, from wobbling cones, to no motion(when in real life there’s plenty of excursion).

It’s just like a stroboscope. Same reason wheels on cars can appear to go backwards etc.

In regards to the water moving backwards, it’s called aliasing error. If you sample audio the same way you get ring modulation, or creation of frequencies that didn’t previously exist. Or if you film a wheel spinning, at certain frequencies it can appear to be spinning backwards, even though it is not….

sorry mike, you’re wrong about how this works. the hose is taped to the speaker and so vibrates at 24Hz. it’s the wiggling motion of the hose that creates the stream – the hose just appears to be stationary because every time the camera captures a frame, the hose is in the same place.

I Made my own version of it a 30fps instead of 24. You get a different effect but still pretty awesome. In this you DO see the hose moving. Probably would have looked better recording the hose. But hay it was fun :)